The present invention relates to a color image sensing apparatus and, more particularly, to a color image sensing apparatus using a single solid-state image sensing element or image pickup tube.
In order to realize a color image sensing apparatus using a single image pickup tube or solid-state image sensing element, a color separation filter, in which a plurality of colors of filter elements are arranged in a mosaic or stripe form, must be provided on an image sensing surface, so as to separate an image signal of one frame into a plurality of color component signals. Two types of color separation filters are known: a primary color separation filter using a combination of the primary colors of red (R), green (G), and blue (B), and a complementary color separation filter using a combination of complementary colors of yellow (Ye: transmitting R and G) and cyan (Cy: transmitting B and G), and white (W: transmitting R, G, and B; in other words, transparent) or G. Since the latter type filter has a higher transmittance than that of the former type, it has the advantages of high sensitivity and resolution.
As a conventional single-plate color image sensing apparatus using the complementary color separation filter, an apparatus described in "A Single Chip CCD Color Camera System Using Field Integration Mode" (Sone, Ishikawa, Hashimoto, Kuroda, Ohkubo) Nippon Television Gakkai-Shi, Vol. 37, No. 10 (1983), pp. 856-862, is known. In the field accumulation mode, signal charges are accumulated over one field period (1/60 sec) by a single photodiode. This mode can eliminate an afterimage and flickering, in contrast with a frame accumulation mode in which signal charges are accumulated over one frame period (1/30 sec) by a single photodiode.
FIG. 1 is a block diagram of the prior art apparatus. An optical image of an object incident through image sensing lens 6 is incident on an image sensing surface of CCD (Charge Coupled Device) 10, as a solid-state image sensing element, through mosaic filter 8 as a color separation filter.
FIG. 2 shows the structure of mosaic filter 8. Mosaic filter 8 is a complementary color filter comprising combinations of 2.times.4 filter elements of cyan-yellow, magenta-green, yellow-cyan; and magenta-green. With this structure, pixel data (data of two adjacent pixels in the vertical direction) for two lines (horizontal lines) are read as an image signal for one horizontal scanning line, and the two horizontal lines for A (first or odd) and B (second or even) fields are shifted by one line in the vertical direction. The arrangement of the pixels of the mosaic filter is determined so that the same luminance signals are obtained for each field and each line, and at least two different color component data can be obtained from one horizontal scanning line if two adjacent pixels in the vertical direction are read as one pixel. The reason why the arrangement in the horizontal direction is repeated for every two columns is to maintain the frequency band of a luminance signal.
For this purpose, a luminance signal consisting of the following color components is output as a horizontal scanning line signal: ##EQU1##
An output signal from CCD 10 is supplied to low-pass filters (LPFs) 12 and 14 and to bandpass filter (BPF) 16. Pass bands of LPFs 12 and 14 are respectively set to be 3 MHz and 0.5 MHz. BPF 16 has a central frequency of 3.58 MHz and a bandwidth of about 1 MHz. Wide-band luminance signal YH and and narrow-band luminance signal YL are respectively output from LPFs 12 and 14.
Luminance signal YH is supplied to video signal processor 20 via .gamma. compensation circuit 18.
Luminance signal YL is supplied to the second input terminals of addition/subtraction circuits 30 and 32.
The output from BPF 16 is supplied to demodulator 22, and is also supplied to demodulator 28 via one-horizontal-scanning-period (1H) delay line 26. A glass delay play utilizing ultrasonic vibration is adopted as 1H delay line 26. Demodulators 22 and 28 subtract odd column signals from even column signals. Thus, demodulator 22 alternately outputs the following color signal as a horizontal scanning line signal for every line.
In the nth line in FIG. 2, the following color signal can be obtained: ##EQU2##
In the (n+1)th line in FIG. 2, the following color signals can be obtained: ##EQU3##
Since the input signal to demodulator 22 is input to demodulator 28, to be delayed by 1H, demodulator 28 outputs color signal 2R-G in the nth line and color signal 2B-G in the (n+1)th line, contrarily to demodulator 22.
The outputs from demodulators 22 and 28 are supplied to line selector 24. Line selector 24 has two input terminals and two output terminals, and alternately outputs first and second input signals from first and second output terminals for every 1H. As a result, the first output terminal of line selector 24 always outputs color signal 2R-G, and the second output terminal always outputs color signal 2B-G.
First output signal 2R-G is supplied to the first input terminal of addition/subtraction circuit 30, and second output signal 2B-G is supplied to the first input terminal of addition/subtraction circuit 32. Circuits 30 and 32 add or subtract luminance signal YL and these color signals, after multiplying with suitable coefficients, and respectively generate color difference signals R-Y and B-Y. The outputs from circuits 30 and 32 are supplied to modulator 34, and are modulated at a central frequency of 3.58 MHz, thereby outputting a color sub-carrier signal. The color sub-carrier signal is supplied to video processor 20.
Video processor 20 produces a composite signal based on the color sub-carrier signal, luminance signal YH output from circuit 18, and a sync signal.
As has been described above, in the conventional apparatus, first and second color signals 2R-G and 2B-G are alternately output for every 1H from demodulator 22 or 28. 1H delay line 26 must be used so that both the first and second signals can be output for every 1H.
Since the glass delay plate is used as 1H delay line 26, a signal at a frequency other than the predetermined central frequency (in this case, 3.58 MHz) cannot be accurately delayed by 1H.
Recently, various types of solid-state image sensing elements having different numbers of pixels have been developed. If the number of pixels is changed, the demodulation frequency of a color signal is also changed, and hence, 1H (time) is changed accordingly. However, suitable glass delay plates are rarely available.